Field of the Invention
The present invention relates to high strength, low yield ratio and high ductility hot or cold rolled dual-phase structure steel sheet having excellent formability.
The low yield ratio used herein means the yield strength/tensile strength ratio which is about 0.6 or less, and the dual-phase structure used herein means a structure in which the main metallographic constituents are ferrite and a transformed phase produced by rapid cooling (such as martensite, or martensite plus bainite including some retained austenite).
In recent years, great efforts have been made in weight reduction of automobile cars mainly motivated by necessity of saving the fuel consumption.
As the thickness of steel materials used in automobile cars is decreased to reduce the weight, it becomes necessary to use a high strength steel in order to assure a satisfactory strength of the automobile cars.
Conventional high strength steels, however, have been limited in their applications due to their drawbacks such that they are confronted with the problem of "spring-back" during their press forming as they have an excessively high yield ratio and that as their work hardening rate (n value) is relatively low, localized strain-concentration takes place early during deformation (namely necking is caused), resulting in crackings.
Meanwhile, the present inventors developed high strength steel sheets free from yield elongation, with a maximum yield ratio (yield strength/tensile strength) of about 0.6, and excellent ductility as disclosed in Japanese Patent Laid-Open Specifications Sho 50-39210 and Sho 51-78730 (and related U.S. Pat. No. 4,062,700).
The steel sheets disclosed in the above Japanese Patent Laid-Open Specifications show a markedly lower yield ratio than the conventional high strength steels, as schematically shown by their stress-strain curves in FIG. 1 (this means less tendency to spring-back), and large work-hardening rate (n value) and elongation (thus less susceptible to cracking), and they can provide high yield strength when given slight strain (this means a high yield strength after forming) as apparently shown in FIG. 1. For these remarkable advantages in press forming, these steel grades are expected to be increasingly used. These steel grades are of dual-phase structure mixed with the ferrite phase and the transformation phase produced by rapid cooling (hereinafter "rapidly cooled transformation phase"), and their maximum limit of yield ratio demanded by users is 0.6.
Now the prior inventions made by the present inventors and disclosed in the aforementioned Japanese Patent Laid-Open Specifications, relate to a process which comprises continuous annealing of a Si-Mn steel containing about 1% Si and about 1.5% Mn in the two-phase (.alpha.+.gamma.) temperature zone (Sho 50-39210) or a process which comprises continuous annealing of an ordinary steel containing about 0.1 to 0.15% C and about 1.5% Mn in the two-phase (.alpha.+.gamma.) temperature zone, preceded by either (1) pre-annealing of the steel in the two-phase (.alpha.+.gamma.) temperature zone or (2) hot rolling the steel with its finishing temperature maintained in the two-phase (.alpha.+.gamma.) temperature zone and coiling at a desired temperature (Sho 51-78730). The features of the prior inventions, such as the high Si-Mn contents (Sho 50-39210), the pre-annealing in the two-phase temperature zone, or the hot roll finishing in the two-phase temperature zone (Sho 51-78730) are for the purposes of increasing the hardenability of the .gamma. phase formed in the steel during the continuous annealing in the two-phase (.alpha.+.gamma.) temperature zone, and thus resulting in a successful dual-phase structure after the eventual cooling.
In the prior inventions, the conditions of cooling after the continuous annealing are so specified that a relatively slow cooling rate should be applied so as to avoid damages on the ductility and shape of the steel sheet. However, regarding the cooling pattern, namely the cooling curve, these prior inventions are based on an ordinary simple cooling pattern, and do not take any special consideration to the cooling pattern. Further the prior inventions are suitable for obtaining a high strength dual-phase structure steel with a minimum tensile strength of about 60 kg/mm.sup.2 and not suitable for production of steels with tensile strengths of 40 to 50 kg/mm.sup.2 which have been strongly sought for by the automobile industry because these steel grades are usuable in a very wide field of applications.